Method for inputting information into a data storage medium that is optically recordable and readable

ABSTRACT

In a method of entering information into an optically writable and readable data storage medium having a polymer film which can be changed locally by heating in order to store information and to which there is assigned an absorber dye ( 34 ) which is set up to absorb a write beam ( 36 ), at least partly, and to transfer the heat produced in the process, at least partly, locally to the polymer film, first of all the absorber dye is heated locally in a predetermined region of the polymer film in accordance with the information to be entered, specifically by means of a pulsed write beam ( 36 ). After that, the absorption properties of the absorber dye ( 34 ) are changed by means of light, to be specific preferably by means of a fixing beam ( 37 ) which bleaches ( 38 ) the absorber dye ( 34 ) in the written region of the polymer film.

This application is the US national phase of international applicationPCT/EP02/06339, filed in English on 10 Jun. 2002, which designated theU.S. PCT/EP02/06339 claims priority to DE Application No. 101 28 901.4filed 15 Jun. 2001. The entire contents of these applications areincorporated herein by reference.

The invention relates to a method of entering information into anoptically writable and readable data storage medium, and also to the useof a data storage medium in conjunction with such a method.

DE 298 16 802 U1 discloses an optically writable and readable datastorage medium which has a polymer film whose refractive index can bechanged locally by heating. If the polymer film is heated locally withthe aid of a write beam, the change in the refractive index results in achange in the reflecting power (reflectivity) at the point considered.This can be used for storing information. In order to read out theinformation, a read beam is used, which is reflected more intensely atpoints with increased reflectivity, which may be measured in order toacquire the information. The polymer film, which, for example, consistsof polypropylene, can be prestressed (stretched) in both surfacedirections during production, as a result of which a high amount ofinherent energy is stored in the material. In the event of local heatingby the write beam, in such a configuration of the polymer film, a sharpmaterial change (compaction) takes place as a result of reformation, therefractive index changing in the desired way. In the case of thepreviously known data storage medium, the polymer film can be assignedan absorber dye which preferentially absorbs the write beam and gives upthe heat produced in the process locally to the polymer film. With theaid of an absorber dye, a sufficiently high change in the refractiveindex (for example a change of about 0.2) can be achieved even with arelatively low intensity of the write beam.

In the previously known data storage medium, the polymer film is woundspirally in a plurality of plies onto a core, there being an adhesionlayer between adjacent plies in each case. By focusing the write beam orread beam, the information may be written specifically into apreselected polymer film ply or read out from it.

A main problem with such spirally wound multi-ply data storage media isthe low transmission after a plurality of plies, which is caused by theabsorber dye. In the case of a preferred optical density between 0.30and 0.35 per ply, after 15 plies an optical density of 5 is reached,which corresponds to a transmission of only 0.001%. The optical densityD is a variable which is well suited to characterizing the absorptionbehavior; it is true that D=log(1/T), where T=I/I₀ is the transmissionif the intensity of the incident radiation falls from I₀ to I. At higheroptical densities per ply, a transmission value of 0.001% is alreadyreached at a lower number of transilluminated plies. This makes writingdata into plies lying at a deeper level virtually impossible, since allof the energy of the write beam, which is needed to change the opticalproperties of the polymer film, is already absorbed in the plies lyinghigher up and transilluminated first. Reading data with the aid of theread beam is affected even more detrimentally, since reading out theinformation is carried out in reflection, so that the read beam has totrace a path within the data storage medium which is twice as long asthat traced by the write beam during the writing operation.

In DE 100 28 113, it is proposed to introduce the absorber dye inoriented fashion in the previously known data storage medium. Bychanging the polarization of a laser used to generate the write beam andthe read beam during writing and reading, it is then possible for thelaser beam to be absorbed during writing but not during reading.However, this measure only simplifies the reading operation, while thewriting operation in plies lying at a deeper level is still hindered bythe high optical density.

It is an object of the invention to provide a possible way to achieveacceptable transmission properties in an optically writable and readabledata storage medium with an absorber dye, so that, in particular in thecase of a spirally wound multi-ply data storage medium of theaforementioned type, as far as possible neither the writing operationnor the reading operation in the layers lying at a deeper level ishindered.

This object is achieved by a method of entering information into anoptically writable and readable data storage medium as claimed in claim1 and the use of such a data storage medium as claimed in claim 18.Advantageous refinements of the invention emerge from the subclaims.

The method according to the invention is used for entering informationinto an optically writable and readable data storage medium having apolymer film which can be changed locally by heating in order to storeinformation. The polymer film is assigned an absorber dye which is setup to absorb a write beam, at least partly, and to give up the heatproduced in the process, at least partly, locally to the polymer film. Adata storage medium of this type as such is known, for example from DE298 16 802 U1, DE 100 28 113 or DE 199 35 776 A1. In the method, theabsorber dye is heated locally in accordance with the information to beentered by means of a pulsed write beam in a predetermined region of thepolymer film. According to the invention, the absorption properties ofthe absorber dye are changed by means of light.

As a result of changing the absorption properties of the absorber dye,the write beam and/or a read beam used to read out information during asubsequent writing or reading operation are no longer seriously hinderedby the absorber dye. The method according to the invention is thereforesuitable in particular for data storage media in which the polymer filmis arranged in multiple plies, the write beam being focused onto apreselected polymer film ply or the associated absorber dye whenentering information. If such a data storage medium is, for example,written from outside to inside and the absorption properties of theabsorber dye are correspondingly changed from outside to inside, thedata storage medium has an adequately high transmission for all thefollowing writing and reading operations.

It is preferable if, following the action of the pulsed write beam, theabsorption properties of the absorber dye are changed in thepredetermined region of the polymer film by means of a fixing beam.Preferred absorber dyes can be bleached (see below) and can therefore bebleached with the aid of the fixing beam, so that they do not hinder thetransmission of the write beam or a read beam. A focused laser beam inthe continuous wave mode may be used as a fixing beam.

Thus, for example, in an advantageous refinement of the method, a pulsedwrite beam focused onto a diameter of 0.5 μm, for example, is used. Thewrite beam heats the absorber dye in a predetermined region of thepolymer film used for information storage (for example in a preselectedtrack in a preselected polymer film ply of a multi-ply data storagemedium). The absorber dye, which, for example, is contained in thepolymer film but can also be arranged in a separate layer on the polymerfilm, transfers the heat to the polymer film, which is changed locallyas a result (for example in refractive index and/or in reflectivebehavior). The fixing beam follows the write beam, here a focused laserbeam in the continuous wave (CW) mode, somewhat wider than the writebeam, which bleaches the absorber dye no longer needed track by track inthe region considered after the writing operation, that is to sayremoves the absorber dye, so to speak, like a rubber eraser. No highrequirements are placed on the fixing beam in terms of quality. The beamprofile and the stability are of subordinate importance; it is merelynecessary to ensure that the power is sufficient to bleach the absorberdye. Furthermore, the fixing beam should have no detrimental effect onthe absorber dye in regions (for example adjacent tracks or polymer filmplies) which are subsequently still to be written.

The designation “fixing beam” is selected in analogy with a photographicprocess in which, in the fixing operation, excess light-sensitivematerial is removed from a photographic film. This analogy illustrates afurther advantage of the method according to the invention: changing theabsorption properties of the absorber dye prevents subsequentinadvertent changing of the information entered.

As already mentioned, in a preferred refinement of the invention, theabsorber dye can be bleached and it is at least partly bleached in orderto change its absorption properties. Absorber dyes which areparticularly suitable for this purpose are dyes which are notparticularly lightfast and can therefore be bleached easily. Typicalexamples of such dyes are azo and diazo dyes. Under the action of light,they are subject to continual cis-trans isomerization, which ultimatelyleads to destruction of the chromophoric system. In addition, the cyaninand phthalocyanin dyes known from CD-ROM technology can be bleachedrelatively easily.

In addition to dyes which can be bleached, photochromic andthermochromic dyes are also suitable as an absorber dye. Photochromicdyes, such as spiro compounds, change their structure and thereforetheir color under irradiation of light of suitable wavelength. If,following the conversion, the compound has a color which is no longerabsorbed by the write beam and/or read beam, the purpose is fulfilled inexactly the same way as in the case of a bleachable dye.

In the case of thermochromic compounds, a structure change and thereforea color change occurs as a result of heating. When the method is carriedout, this color change is achieved by the absorber dye absorbing thefixing beam and converting its light energy into heat energy.

The changes in the case of thermochromic and photochromic dyes are oftenreversible, which can be advantageous for specific applications.

In general, it is true that the write beam must heat the polymer filmlocally in order to enter information, specifically via the associatedabsorber dye. The write beam is therefore operated in the pulsed mode(for example with short pulses of less than about 1 μs duration). In thecase of the action of the fixing beam on the absorber dye, on the otherhand, the associated region of the polymer film should not be changed,that is to say this region may be heated only slightly. For the fixingbeam, therefore, a laser beam in the continuous wave mode which has alower intensity than the write beam is suitable.

Apart from the application of a fixing beam to a predetermined region ofthe polymer film, preferably directly following the entering ofinformation into this region with the aid of the write beam, there isalso the possibility that the absorption properties of the absorber dyein the data storage medium will be changed as a whole, after the actionof the pulsed write beam on the data storage medium has been completed.For this purpose, the data storage medium is, for example, irradiatedover the entire area with ultraviolet light, in order to bleach all ofthe absorber dye contained in the data storage medium. After that, nofurther information can be entered. The low expenditure as compared withthe use of a fixing beam is advantageous. However, there is thedisadvantage that only the reading operations are made easier since,during the writing operations, the absorber dye must still be presentand thus has a detrimental effect on the transmission of the write beam.

If the polymer film in the data storage medium is arranged in amultiplicity of plies, there is preferably in each case an adhesionlayer, which optionally contains absorber dye, between adjacent polymerfilm plies. The refractive index of the adhesion layer preferablydiffers only slightly from the refractive index of the polymer film, inorder that no undesired reflections occur at the interfaces between anadhesion layer and a polymer film ply, which have a detrimental effecton the transmission of the write beam or of the read beam. On the otherhand, small differences between the refractive indices of the polymerfilm plies and the adhesion layers can be utilized for formatting thedata storage medium. The difference in the refractive indices ispreferably so small that the reflection at the interface is below 4% or,even better, below 1%. Particularly beneficial conditions may beachieved if the difference in the refractive indices is less than 0.005.

In the case of a multi-ply data storage medium, the polymer film ispreferably wound spirally. In this way, with the aid of a single polymerfilm, a multi-ply structure of the data storage medium may be achieved,which permits a high storage density and a high storage capacity. Inthis case, the data storage medium preferably has an opticallytransparent core which is set up to accommodate a writing and readingdevice of a drive matched to the data storage medium. The drive can havea writing and/or reading head which moves in the interior of thetransparent core, relative to the data storage medium, which is at rest,or in which the write and/or read beam is coupled into the data storagemedium via moving optical elements. Since, in this case, the datastorage medium itself is at rest, it does not need to be balanced withregard to a rapid rotational movement.

The polymer film used as the data carrier is preferably stretched, forexample by being prestressed in two mutually perpendicular directionswithin its plane during production. This leads to a high energy densitybeing stored in the film material. As a result of the deposition of arelatively low quantity of energy per unit area with the aid of thewrite beam, an intense material change (for example material compaction)can then be obtained by reformation which, for example, results in alocal change in the refractive index and a change in the optical pathlength in the material. The change in the refractive index in the regionwhich is heated locally by the write beam is preferably of the order ofmagnitude of 0.2, which leads to a change in the local reflectance,which may easily be detected with the aid of a read beam.

Preferred materials for the polymer film are polypropylene, polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polymethylpentene(PMP; also poly-2-methylpentene), polyvinylchloride and polyimide. Whilebiaxially oriented polypropylene (BOPP) and polyvinylchloride (PVC) areparticularly cost-effective, the other materials mentioned have acrystallite melting point of at least 170° C., which leads to increasedlong-term stability.

In the following text, the invention will be explained further usingexemplary embodiments. In the drawings:

FIG. 1 shows a data storage medium with a spirally wound polymer filmused for information storage, parts of a drive being arranged in thecentral region of the data storage medium, and

FIG. 2 shows a detail from one ply of the data storage medium, which isillustrated flat for the purpose of illustration, in a schematic planview.

FIG. 1 shows, in a schematic illustration, a data storage medium 1 and awriting and reading device 2 of a drive matched to the data storagemedium 1. The data storage medium 1 has a number of plies 10 of apolymer film 11 used as an information carrier, which is wound spirallyonto an optically transparent core. The sleeve-like core is notillustrated in FIG. 1 for reasons of clarity; it is located inside theinnermost ply 10. For better illustration, the individual plies 10 ofthe polymer film 11 are shown in FIG. 1 as concentric circular rings,although the plies 10 are formed by winding the polymer film 11spirally. In each case an adhesion layer 12 is arranged between adjacentplies 10 of the polymer film 11. For reasons of clarity, the adhesionlayers 12 are drawn with an enlarged thickness in FIG. 1, not to scale.

In the exemplary embodiment, the polymer film 11 consists of biaxiallyoriented polypropylene and has been prestressed in both surfacedirections before being wound. In the exemplary embodiment, the polymerfilm 11 has a thickness of 35 μm; other thicknesses in the range from 10μm to 100 μm or else thicknesses lying outside this range are likewiseconceivable. The adhesion layers 12 are free of gas bubbles and, in theexemplary embodiment, consist of acrylate adhesive with a thickness of23 μm, preferred layer thicknesses being between 1 μm and 40 μm. Insteadof an acrylate adhesive, a varnish, for example, can also be used forthe adhesion layers.

Arranged on the polymer film 11 is an absorber layer, which contains anabsorber dye (see below) which is embedded in a matrix of polymethylmethacrylate (PMMA). The thickness of the absorber layer is preferablyless than 0.5 μm and, in the exemplary embodiment, is 0.4 μm. The wounddata storage medium 1 thus has a layer structure in which in each case aply 10 of the polymer film 11 is followed by an absorber layer and thenan adhesion layer 12.

In the exemplary embodiment, the data storage medium 1 contains twentyplies 10 of the polymer film 11 and has an external diameter of about 30mm. The height of the core is 19 mm. A different number of plies 10 orother dimensions are likewise possible. The number of windings or pliescan lie, for example, between ten and thirty, but can also be greaterthan thirty.

The writing and reading device 2 arranged in the interior of the corecontains a writing and reading head 20 which, with the aid of amechanism 21, can be rotated and moved axially to and fro in thedirections of the arrows shown. The writing and reading head 20 hasoptical elements with the aid of which a light beam (for example with awavelength of 630 nm or 532 nm) produced by a laser not illustrated inFIG. 1 can be focused onto the individual plies 10 of the polymer film11. Since the writing and reading head 20 is moved with the aid of themechanism 21, it is able to scan all the plies 10 of the data storagemedium 1 completely. In the exemplary embodiment, the data storagemedium 1 is at rest. It therefore does not need to be balanced withregard to a high rotational speed (and does not have to be unwound orspooled either). For reasons of clarity, the elements provided forbalancing the writing and reading head 20 are not shown in FIG. 1. Theaforementioned laser is located outside the writing and reading head 20and is stationary; the laser beam is deflected into the writing andreading head 20 by optical elements.

In order to enter or write information into the data storage medium 1,the laser in the exemplary embodiment is operated with a beam power ofabout 10 mW. In this case, the laser beam is used as a write beam and isfocused onto a preselected ply 10 of the polymer film 11 (or theabsorber layer on the polymer film 11 in this ply 10), so that the beamspot is smaller than 1 μm. The light energy is in this case introducedin the form of short pulses of less than 1 μs duration. The energy ofthe write beam is absorbed in the beam spot, primarily by the absorberdye, which leads to local heating of the polymer film 11 and thereforeto a local change in the refractive index and the reflecting behavior.During the writing operation, the write beam is defocused in the pliesadjacent to the ply 10 considered of the polymer film 11, so that theadjacent plies of the polymer film 11 and the associated absorber dyeare locally heated only slightly and the stored information is notchanged there.

In the polymer film 11, the information units are formed by changing theoptical properties in a region with a preferred size of less than 1 μm.In the process, the information can be stored in binary form, that is tosay the local reflectivity assumes only two values at the location of aninformation unit. This means that, if the reflectivity is above adefined threshold value, a “1”, for example, is stored at the consideredlocation of the information carrier, and if it is below this thresholdvalue or below another, lower threshold value, accordingly a “0”.However, it is also conceivable to store the information in a pluralityof gray stages. This is possible if the reflectivity of the polymer filmat the location of an information unit can be changed in a specificmanner by means of defined setting of the refractive index, withoutsaturation being reached in the process.

FIG. 2 is a schematic illustration of a detail 30 of one ply 10 of thepolymer film 11 with the absorber dye contained in the adjacent absorberlayer. For reasons of clarity, the detail 30 is placed in the plane ofthe paper, although the polymer film 11 in the data storage medium 1 iswound and is therefore curved.

With the aid of the write beam, the information units are entered alongpreformatted tracks 32. For this purpose, the write beam, as alreadyexplained, is focused onto the polymer film 11 or the absorber layerarranged on the latter. This leads to local heating of the absorber dye34, shown hatched in FIG. 2, and therefore to a local change of thepolymer film 11. The region of the focus of the write beam is designated36 in FIG. 2. Its diameter is preferably about 0.5 μm.

According to the invention, the absorption properties of the absorberdye 34 are changed after the desired information units have been writteninto a predetermined region of the polymer film 11 (the upper track 32here). For this purpose, use is made in the exemplary embodiment of afixing beam 37 which bleaches the absorber dye 34. This thereforeproduces a bleached region 38, in which the absorber dye no longerabsorbs the write beam and a read beam or at least does so only to aconsiderably lesser extent.

In the exemplary embodiment, the fixing beam 37 has a larger region ofthe focus than the write beam 36, as can be seen in FIG. 2. Accordingly,the intensity (power per unit area) is lower. In the exemplaryembodiment, the fixing beam 37 follows the write beam 36, that is tosay, in the illustration according to FIG. 2, the track 32 is writtenfrom left to right with the aid of the write beam 36 and, immediatelythereafter, the fixing beam 37 bleaches the absorber dye 34. In theexemplary embodiment, the fixing beam 37 is produced by a separatelaser, which is operated in the continuous wave mode (CW mode) with apower of 10 mW and at a wavelength of 650 nm. The writing and readinghead 20 is also used to inject the fixing beam 37; however, detailsrelating to this are not shown in the schematic illustration accordingto FIG. 1.

Alternatively, it is conceivable to use the same laser and the sameoptics both for the write beam and for the fixing beam. For thispurpose, for example, the laser is firstly operated in a pulsed mannerto provide the write beam, in order to enter the desired informationinto a predetermined region of the polymer film 11 (for example a trackor a section of a track). After that, the laser is switched into CW modeand the laser beam is defocused somewhat, in order to move over thisregion again as a fixing beam and in the process to bleach the absorberdye.

In the exemplary embodiment, the absorber dye is “Fettschwarz[literally: grease black] (Fluka 46300)”. This is an azo dye which mayeasily be bleached. The absorber dye is contained in the absorber layerin the exemplary embodiment, specifically in a concentration such that,with the given thickness of the absorber layer, the result is an opticaldensity of about 0.3 at the light wavelength of the write beam. Theoptical density can be lower, but also higher, since the absorber dyedoes not hinder the transmission of the write beam or of a read beamfurther if the data storage medium is, for example, written from insideto outside and the excess absorber dye is bleached, as explained, aftera writing operation.

The optical density is a variable which is well suited to characterizingthe absorption behavior. For the optical density D, it is true that:D=log(1/T)=ε_(λ) c d

Here, T=I/I₀ is the transmission through a layer of thickness d, theintensity of the incident radiation falling from I₀ to I, ε_(λ) is theextinction coefficient at the wavelength λ used(concentration-independent material parameter), and c is theconcentration of the absorber dye.

In order to read stored information out of the data storage medium 1,the laser used for the write beam in the exemplary embodiment isoperated in the CW mode. Depending on the stored information, the readbeam focused onto the desired location is reflected, and the intensityof the reflected beam is registered by a detector in the writing andreading device 2.

1. A method of entering information into an optically writable andreadable data storage medium having a polymer film which can be changedlocally by heating in order to store information and to which there isassigned an absorber dye which is set up to absorb a write beam, atleast partly, and to transfer the heat produced in the process, at leastpartly, locally to the polymer film, comprising the steps of: (1) localheating of the absorber dye in accordance with the information to beentered by means of a pulsed write beam in a predetermined region of thepolymer film, (2) changing the absorption properties of the absorber dyeby means of light.
 2. The method as claimed in claim 1, wherein,following the action of the pulsed write beam, the absorption propertiesof the absorber dye are changed in the predetermined region of thepolymer film by means of a fixing beam.
 3. The method as claimed inclaim 2, wherein the fixing beam used is a focused laser beam in thecontinuous wave mode.
 4. The method as claimed in claim 1, wherein,after the conclusion of the action of the pulsed write beam on the datastorage medium, the absorption properties of the absorber dye in thedata storage medium are changed as a whole.
 5. The method as claimed inclaim 1, wherein the absorber dye can be bleached and is at least partlybleached in order to change its absorption properties.
 6. The method asclaimed in claim 1, wherein the absorber dye is photochromic and itscolor is changed when irradiated with light.
 7. The method as claimed inclaim 1, wherein the absorber dye is thermochromic and is heated whenirradiated with light, its color being changed.
 8. The method as claimedin claim 1, wherein the polymer film contains absorber dye.
 9. Themethod as claimed in claim 1, wherein a layer which contains absorberdye is arranged on the polymer film.
 10. The method as claimed in claim1, wherein the polymer film is arranged in the data storage medium in aplurality of plies, and the write beam is focused onto a preselectedpolymer film ply or the associated absorber dye when information isbeing entered.
 11. The method as claimed in claim 10, wherein anadhesion layer which optionally contains absorber dye is arrangedbetween adjacent polymer film plies in each case.
 12. The method asclaimed in claim 11, wherein the refractive index of the adhesion layerdiffers only slightly from the refractive index of the polymer film. 13.The method as claimed in claim 10, wherein the polymer film is woundspirally.
 14. The method as claimed in claim 13, wherein the datastorage medium has an optically transparent core which accommodates awriting and reading device of a drive matched to the data storagemedium.
 15. The method as claimed in claim 1, wherein the refractiveindex of the polymer film is changed locally by heating.
 16. The methodas claimed in claim 1, wherein the polymer film is stretched.
 17. Themethod as claimed in claim 1, wherein the polymer film has a materialwhich is selected from the following group: polypropylene, polyethyleneterephthalate, polyethylene naphthalate, polymethylpentene,polyvinylchloride, polyimide.
 18. The use of an optically writable andreadable data storage medium having a polymer film which can be changedlocally by heating in order to store information and to which there isassigned an absorber dye which is set up to absorb a write beam, atleast partly, and to transfer the heat produced in the process, at leastpartly, locally to the polymer film, in a drive in which, in order toenter information into the data storage medium, the absorber dye isheated locally in a predetermined region of the polymer film by means ofa pulsed write beam in accordance with the information to be enteredand, after that, the absorption properties of the absorber dye arechanged by means of light.
 19. The use as claimed in claim 18, the datastorage medium and, respectively, the method carried out with the aid ofthe drive for entering information into the data storage mediumadditionally having the features as claimed in claim
 2. 20. The methodas claimed in claim 4, wherein the absorption properties of the absorberdye are changed by full-area irradiation with UV light.
 21. The methodas claimed in claim 16, wherein the polymer film is biaxially stretched.